1. Field of the Invention
The present invention relates generally to a lithium secondary battery provided with a positive electrode, a negative electrode, and a non-aqueous electrolyte, and more particularly, to a lithium secondary battery whose charge/discharge cycle performance is improved upon improvement of a positive electrode active material used for its positive electrode or a negative electrode active material used for its negative electrode.
2. Description of the Related Art
In recent years, secondary batteries have begun to be used in various fields such as electronic equipment and the like. Lithium secondary batteries are attracting great attention as one of new-type batteries having high power and high energy density, and various efforts have been made to develop such lithium secondary batteries.
In order to improve charge/discharge cycle performance of such lithium secondary batteries, one so adapted as to employ as a negative electrode active material an oxide such as tungsten trioxide which is previously lithiumized has been proposed, as disclosed in Japanese Patent Laid-Open No. Hei8(1996)-241707.
Unfortunately, however, the above-mentioned tungsten trioxide has an unstable crystal structure, as described in the literature (J. Guo, Y. J. Li and M. S. Whittingham, J. Power. Sources., 54, 461, 1995). When the tungsten trioxide is used as a negative electrode active material in a lithium secondary battery, the tungsten trioxide is degraded in capacity of occluding and discharging lithium due to the transformation of its crystal structure. Accordingly, there still exist a problem that the battery cannot attain an adequate improvement in charge/discharge cycle performance.
An object of the present invention is to improve charge/discharge cycle performance of a lithium secondary battery provided with a positive electrode, a negative electrode, and a non-aqueous electrolyte upon improvement of a positive electrode active material used for said positive electrode or a negative electrode active material used for said negative electrode.
A lithium secondary battery according to the present invention is a lithium secondary battery provided with a positive electrode, a negative electrode, and a non-aqueous electrolyte, wherein a composite oxide represented by a chemical formula MXW1-XOY (wherein M denotes at least one type of metal element selected from the group consisting of Cu, V, Cr, Mn, Fe, Co, and Ni; and the conditions of 0 less than X less than 0.46 and 2.5xe2x89xa6Yxe2x89xa63.5 are satisfied) and having a monoclinic crystal structure or the composite oxide to which lithium is added is used as a positive electrode active material for said positive electrode or a negative electrode active material for said negative electrode.
The literature (Binary Alloy Phase Diagrams, (1986), American Society for Metals: M-O binary phase diagram) shows that the metal element M, which is selected from the group consisting of Cu, V, Cr, Mn, Fe, Co, and Ni, in the composite oxide represented by the foregoing chemical formula forms a stable compound whose decomposition temperature is more than 1000xc2x0 C. in combination with an oxygen atom O.
When the metal element M is added to tungsten trioxide to obtain a composite oxide represented by the foregoing chemical formula, the composite oxide has a monoclinic crystal structure similar to that of the tungsten trioxide. In addition, the metal elements M are incorporated in some of crystal lattices of the tungsten dioxide to attain a relatively strong chemical bond with oxygen atoms O, thereby stabilizing the crystal structure of the composite oxide.
Accordingly, when the composite oxide represented by the foregoing chemical formula is used as a positive electrode active material or a negative electrode active material in a lithium secondary battery, the composite oxide is prevented from being degraded in capacity of occluding and discharging lithium due to the transformation of its crystal structure. The lithium secondary battery excellent in charge/discharge cycle performance thus can be obtained.
Further, a lithium secondary battery employing a composite oxide represented by a chemical formula MXW1-XOY (wherein M denotes at least one type of metal element selected from the group consisting of Cu, V, Cr, Mn, Fe, Co, and Ni; and the conditions of 0.02xe2x89xa6Xxe2x89xa60.45 and 2.5xe2x89xa6Yxe2x89xa63.5 are satisfied) and having a monoclinic crystal structure as a positive electrode active material or a negative electrode active material can attain more excellent charge/discharge cycle performance because the crystal structure of the composite oxide is further stabilized.
When other element such as Cd, La, Ce, Sm, or Mo, which forms highly stable compound in combination with an oxygen atom O as described above, is used as the metal element M in the above-mentioned composite oxide, the resultant composite oxide is still expected to be effective in improving charge/discharge cycle performance of a lithium secondary battery.
When the composite oxide represented by the foregoing chemical formula is used as a positive electrode active material in the lithium secondary battery of the present invention, various materials generally used in lithium secondary batteries may be used as a negative electrode active material. Examples of a usable material include carbon materials capable of electrochemically occluding and discharging lithium, such as natural graphite, artificial graphite, coke, and calcined products of organic substances; lithium alloys such as an Lixe2x80x94Al alloy, an Lixe2x80x94Mg alloy, an Lixe2x80x94In alloy, and an Lixe2x80x94Alxe2x80x94Mn alloy; and lithium metals. However, when the Li alloys or Li metals is used as the negative electrode active material, branch-like dendrite crystals grow during the charging and discharging of the battery so that a short circuit may occur in the battery. Therefore, it is preferable to use the carbon materials as the negative electrode active material.
On the other hand, when the composite oxide represented by the foregoing chemical formula is used as a negative electrode active material in the lithium secondary battery of the present invention, various materials generally used in lithium secondary batteries may be used as a positive electrode active material. When a lithium-containing transition metal oxide such as LiCoO2, LiNiO2, LiMn2O4, LiMnO2, LiCo0.5Ni0.5O2, LiNi0.7CO0.2Mn0.1O2, LiCo0.9Ti0.1O2, or LiCo0.5Ni0.4Zr0.1O2 is used as the positive electrode active material, the lithium secondary battery with a charge voltage of approximately 1.9 V and a discharge voltage of approximately 1.1 V is obtained.
Then, a case where the composite oxide represented by the foregoing chemical formula is used as the positive electrode active material and a case where the composite oxide represented by the foregoing chemical formula is used as the negative electrode active material are compared with each other. As a result, it was found that the lithium secondary battery employing the composite oxide as the positive electrode active material requires higher charging voltage, whereby the non-aqueous electrolyte solution is liable to be decomposed. Accordingly, it is preferable to use the composite oxide represented by the forgoing chemical formula as the negative electrode active material.
Further, in the lithium secondary battery according to the present invention, the above-mentioned composite oxide represented by the forgoing chemical formula can be synthesized by calcining each element to compose the composite oxide, a compound containing the element, and the mixture of these.
When they are calcined at temperatures of less than 400xc2x0 C., the above-mentioned metal element M may not be sufficiently dispersed in crystal lattices of tungsten trioxide. On the other hand, when they are calcined at high temperatures of more than 1400xc2x0 C., calcined products melt, resulting in uneven composition of the composite oxide when they are cooled down to room temperature, as shown in W-O binary phase diagram in the above-mentioned reference (Binary Alloy Phase Diagrams, Vol.2, p1798 (1986), American Society for Metals). Accordingly, when the lithium secondary battery employs as the positive electrode active material or the negative electrode active material the composite oxide calcined at temperatures of less than 400xc2x0 C. or more than 1400xc2x0 C., it is difficult to sufficiently improved the charge/discharge cycle performance of the battery. Therefore, the composite oxide represented by the foregoing chemical formula is preferably obtained by being calcined at temperatures of not less than 400xc2x0 C. and not more than 1400xc2x0 C., and more preferably not less than 600xc2x0 C. and not more than 1200xc2x0 C.
The lithium secondary battery according to the present invention is characterized in that the composite oxide represented by the foregoing chemical formula is used as a positive electrode active material or a negative electrode active material. It is to be noted that a non-aqueous electrolyte used in the lithium secondary battery is not particularly limited and any known non-aqueous electrolytes generally utilized may be employed.
As such a non-aqueous electrolyte, a non-aqueous electrolyte solution obtained by dissolving a solute in an organic solvent or a solid electrolyte may be used.
Examples of an organic solvent to be used in the non-aqueous electrolyte solution include cyclic carbonic esters such as ethylene carbonate, propylene carbonate, vinylene carbonate, and butylene carbonate; chain carbonic esters such as dimethyl carbonate, diethyl carbonate, and methyl ethyl carbonate; sulfolane; tetrahydrofuran; 1,3-dioxolane; 1,2-diethoxyethane; 1,2-dimethoxyethane; ethoxymethoxyethane; and the like. These solvents may be used alone or in combination of two or more types.
Further, examples of a solute to be dissolved in the above-mentioned organic solvent include lithium compounds such as LiPF6, LiBF4, LiCF3SO3, LiN(CF3SO2)2, LiN(C2F5SO2)2, LiN(CF3SO2)(C4F9SO2), LiC(CF3SO2)3, and LiC(C2F5SO2)3.
Furthermore, examples of a usable solid electrolyte include a polymer electrolyte comprising a polymer such as polyethylene oxide or polyacrylonitrile containing the above-mentioned solute therein, a gelled polymer electrolyte comprising the above-mentioned polymer impregnated with the above-mentioned non-aqueous electrolyte solution, and an inorganic solid electrolyte such as LiI and Li3N.
These and other objects, advantages and features of the invention will become apparent from the following description thereof taken in conjunction with the accompanying drawings which illustrate specific embodiment of the invention.